1. Field of the Invention
The present invention relates to an improved downhole jar apparatus that delivers upward blows, and which is activated by pumping an entire dart valve assembly downhole through a tubing string or work string to land in a vibratory tool body assembly.
2. Background Art
In downhole well operation, there is often a need to perform longitudinal jarring, or vibratory impact, operations. For example, such a jarring operation is often used during work-over operations, using a pipe string or work string such as a coil tubing unit or snubbing equipment. In particular, during fishing operations, it is sometimes necessary to apply upward jarring forces near the bottom of the work string, if the fishing tool becomes stuck.
It is known to operate a jarring device by fluid pressure acting on a dart valve and piston, to urge the dart valve and piston longitudinally in the downhole direction until the downhole movement of the dart valve is stopped. When the downhole movement of the dart valve stops, the seal between the dart valve and the piston is broken, and both the dart valve and the piston move longitudinally in the uphole direction. That is, when the dart valve unseats, the dart valve and the piston both move uphole under spring pressure. This causes the piston to strike some sort of downwardly facing anvil surface in the housing of the tool. It is also known to activate the dart valve mechanism by pumping a device such as a ball downhole, to plug a fluid flow path in the dart valve assembly.
In all such known devices, both the piston and at least part of the dart valve assembly are permanent components of the jarring apparatus, which is installed as a permanent part of the work string. Because the dart valve operates by stopping fluid flow through the jarring apparatus, its permanently installed mechanism necessarily occupies a substantial portion of the inner bore of the jarring apparatus. This presence of the dart valve mechanism in the inner bore of the apparatus necessarily restricts access through the dart valve portion of the apparatus, which can prevent the performance of some operations below the jarring tool, such as free-point measurements, gravel packing operations, drilling operations, fishing operations, or other wireline or coil tubing operations.
For example, U.S. Pat. No. 3,361,220 to Brown discloses a dart valve assembly consisting of a drop-in dart valve V and a valve seat 12b in the jarring mechanism A. The jarring mechanism A, which includes the valve seat, is permanently installed in the work string. The valve seat, considerably smaller than the bore of the work string, restricts access through the jarring apparatus to any portion of the well bore below the tool. It should also be noted that the drop-in portion V of the dart valve assembly is not latched into the tool, leaving it free to fly upwardly an undefined distance at each stroke of the tool. Further, it should be noted that the dart valve V does not have a limited downward travel, so it simply rides downwardly with the piston 12 until the piston bottoms out, or until the fluid pressure equalizes against the piston return spring pressure. This increases wear on the piston return spring and reduces the predictability of each valve cycle.
The existence of a sufficiently open bore through the jarring apparatus to allow the performance of the aforementioned types of operations through the bore is a goal of the present invention. As used herein, an “open bore” through the apparatus should be understood to mean that the inner bore of the dart valve portion of the jarring apparatus is unrestricted, at least to a diameter matching the diameter of the inner bore through the piston portion of the jarring apparatus.
Therefore, it is desirable to have a jarring apparatus which can be installed in the work string with essentially an open bore, but which can be converted to an effective jarring apparatus when required. It is also desirable to be able to return the jarring apparatus to the open bore condition after performance of the jarring operation.